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    <title>Document</title>
    <script>
        // 1.一个变量自己加1
        var num = 1;
        num = num + 1; //++num 或 num++
        num = num - 1; //--num
        console.log(num);

        // 后置自增 是先返回原值 后自加1
        var age = 10,
            p = 10;
        console.log(age++ + 10); //20
        console.log(++p + 10); //21


        var a = 10;
        ++a; //++a a=11
        var b = ++a + 2; //++a = 12
        console.log(b); //14

        var c = 10;
        c++; //c=11
        var d = c++ + 2; //c++=11  c=12
        console.log(d); //13

        var e = 10;
        var f = e++ + ++e; //e++=10 e=11 ++e=12
        console.log(f); //22
    </script>
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